High strength aluminum based alloy and the article made thereof

ABSTRACT

The invention relates to high strength aluminium—based alloy of Al—Zn—Mg—Cu system and the articles made thereof. The present alloy is characterized by the combination of improved properties: flowability, technological plasticity, fracture toughness while preserving high levels of strength properties. 
     Said alloy comprises (mass. %): 
     
       
         
               
               
               
               
               
             
                   
                   
               
                   
                 Zn 
                 6.35-8.0 
                 Si 
                 0.01-0.2 
               
                   
                 Mg 
                 0.5-2.5 
                 Fe 
                 0.06-0.25 
               
                   
                 Cu 
                 0.8-1.3 
                 Zr 
                 0.07-0.2 
               
                   
                 Cr 
                 0.001-0.05 
                 Ti 
                 0.03-0.1 
               
                   
                 Mn 
                 0.001-0.1 
                 Be 
                 0.0001-0.05 
               
                   
                   
               
           
              
             
             
              
              
              
              
              
              
             
          
         
       
     
     and at least one element from the group of alkali-earth metals: 
     
       
         
               
               
               
             
                   
                   
               
                   
                 K 
                 0.0001-0.01 
               
                   
                 Na 
                 0.0001-0.01 
               
                   
                 Ca 
                 0.0001-0.01 
               
               
               
             
                   
                 Al-balance 
               
                   
                   
               
           
              
             
             
              
              
              
             
          
           
              
              
             
          
         
       
     
     the sum Zr+2Ti≦0.3%, and the ratio Si:Be≧2. 
     The invented alloy is suitable for producing large-sized, extruded, rolled and forged semiproduct articles used for manufacture of loaded members of aircraft, cars and other machinery.

FIELD OF THE INVENTION

This invention relates to non-ferrous metallurgy, and in particular itrelates to high strength aluminium based alloys of Al—Zn—Mg—Cu system.The invented alloy is suitable for producing extruded, rolled and forgedsemiproducts (mainly articles having large sections) used formanufacture loaded members of aircraft, lorries and cars, seagoing andriver vessels, agricultural machinery.

BACKGROUND OF THE INVENTION

Al—Zn—Mg—Cu alloys are widely used in the aircraft and aerospaceindustries. Well-known is the Russian alloy of said system comprising(mass. %):

Zn 6.5-7.3 Fe 0.2-0.4 Mg 1.6-2.2 Si < 0.2 Cu 0.8-1.2 Al-balance

Said alloy doesn't provide high (UTS,YTS) properties and fracturetoughness (K_(1c)). The articles made from said alloy have limitedefficiency in weight and unsatisfactory service life (Handbook,Aluminium alloys, 1984, Moscow, publ. “Metallurgy”.

The American alloys of Al—Zn—Mg—Cu system (7000 series) developed byALCOA are also well-known. For instance, the alloy described in U.S.Pat. No. 4,828,631 comprises (in mass. %):

Zn 5.9-8.2 Ti < 0.06 Mg 1.5-4.0 Si < 0.12 Cu 1.5-3.0 Fe < 0.15 Zr0.08-0.15 impurities < 0.05 each and < 0.15 in total B < 0.01 Al-balanceCr < 0.4

This alloy has been developed for particular use in aircraft andaerospace articles. It has superior exfoliation corrosion resistance,but its hardenability is sacrificed. In case the semiproduct has thethickness of more than 100 mm, the service characteristics (fracturetoughness, strength, plasticity, corrosion resistance and uniformity ofproperties in semiproducts' volume) become worse. All these shortcomingsdo not allow to produce large—sized articles from said alloy.

The alloy described in U.S. Pat. No. 4,832,758 comprises (in mass. %):

Zn 4.0-8.0 Mg 1.5-3.0 Cu 1.0-2.5

at least one element from the group:

Cr 0.05-0.3 Mn 0.1-0.5 Zr 0.05-0.3 Al-balance

This alloy is intended to be used for producing semiproducts (plates) ofthe limited thickness (not more than 64 mm) because when increasing thethickness of a semiproduct, its mechanical properties, fracturetoughness and corrosion resistance are essentially reduced.

The alloy desclosed in EP 0829552 comprises (in mass. %):

Zn 5.2-6.8 Si ≦ 0.06 Mg 1.6-2.1 Fe ≦ 0.06 Cu 1.75-2.4 Fe + Si ≦ 0.11 Zr0.08-0.15 Al-balance

This alloy may be used for manufacture of wing members of jet aircraft,mainly spars, lower skins, etc. The disadvantage of this alloy is its'high sensibility to quenching rate which leads to sharp reducing thestrength and fracture toughness in case the semiproduct has thethickness more than 60 mm. Therefore when irregular-shaped members(fittings, landing gear elements, etc) are to be manufactured from saidalloy, the great difficulties arise in the process of mechanicalworking.

Pechiney of France also has claimed several alloys of Al—Zn—Mg—Cusystem. The alloy described in EP 0391815 comprises (in mass. %):

Zn 5.5-8.45 Si ≦ 0.5 Mg 2.0-3.5 Fe ≦ 0.5 Cu 0.5-2.5 other elements 0.05each but not more Cr 0.3-0.6 than 0.15 in total amount Mn 0.3-1.1Al-balance

This alloy is intended to be used for producing small-sized semiproducts(sheets, plates, extruded articles) prepared by powder metallurgymethod.

The disadvantage of such products is the low level of fracture toughness(K_(1c)) and low technological properties.

The high strength Al alloy described in PCT/FR 97/00144 comprises (inmass. %):

Zn 5.9-8.7 Si < 0.11 Mg 1.7-2.5 Fe < 0.14 Cu 1.4-2.2 Zr 0.05-0.15 Cr <0.02 Mg + Cu < 4.1 Mn < 0.02 Al-balance

The technological properties (flowability, technological plasticity) ofthis alloy are insufficient, and besides it has a reduced level offracture toughness (K_(1c)).

The articles made from this alloy (i.e. fittings, frames) havenon-uniform strength properties and fracture toughness upon thicknessparticularly in case of large sections.

DESCRIPTION OF THE INVENTION

The object of the present invention is to provide aluminium-based alloyof Al—Zn—Mg—Cu system having the improved combination of properties suchas flowability, technological plasticity, increased fracture toughness,and also ensuring the uniformity of mechanical properties and fracturetoughness upon product's thickness while preserving high levels ofstrength properties, and to provide the articles made from said alloywith said properties.

Accordingly, there is provided Al—Zn—Mg—Cu alloy comprising (in mass.%):

Zn 6.35-8.0 Si 0.01-0.2 Mg 0.5-2.5 Fe 0.06-0.25 Cu 0.8-1.3 Zr 0.07-0.2Cr 0.001-0.05 Ti 0.03-0.1 Mn 0.001-0.1 Be 0.0001-0.05

at least one element from the group consisting of alkali-earth metals:

K 0.0001-0.01 Na 0.0001-0.01 Ca 0.0001-0.01 Al-balance

Zr+2Ti≦0.3 andSi:Be≦2

and the article made thereof.

Alloying of the claimed alloy with additional elements—Be and at leastone element from the group consisting of alkali-earth metals—K, Na, Ca,leads to increase in melt flowability upon casting due to theirinteraction with blisters and hydrogen being present in the metal, whichin turn allows to perform melt filtration and degassing moreeffectively, that means to increase its purity and, as a result, toimprove the technological plasticity of ingots.

The optimum ratio of Zr and Ti combined with lower amount of Cu and inpresence of at least one of the alkali-earth metals—K, Na, Ca, provideimproved level of fracture toughness while preserving high level ofstrength properties due to the reduction of volume content of primaryphases and their refining, and also provide great uniformity ofmechanical properties and fracture toughness upon product's thicknessowing to more uniform distribution of secondary phases' particles inmicrograin's volume, which ensures better hardenability of the presentalloy.

Embodiments of the present invention will now be described by way ofexample.

EXAMPLE

For the purpose of the experiments, the ingots were cast from thealloys, the compositions of which are given in Table 1.

The alloys 2-9 are embodiments of the present invention (the presentalloys or the claimed alloys), and the alloy 1—invention of PCT/FR97/00144.

The hand forgings of, 60, 100, 150, 200 mm thickness (t) were made fromhomogenized ingots by the method of upsetting on a vertical press andthe strips of 50 and 130 mm thickness (t) were made by extrusion on ahorizontal press.

Semiproducts were heat treated as follows: solution heat treatment attemperature of 470° C., time (depending upon semiproduct's thickness)varied from 1 to 3 hours; and water-quenching; two step aging: attemperature 115° C. for 6 hours and at 170° C. for 10 hours.

The alloys flowability was estimated by conventional method by thelength of a straight rod cast into a metallic mold. The technologicalplasticity was estimated by two methods: by upsetting the cylindricalsamples on a press until a side crack appeared, and by tensile testingthe conventional cylindrical samples.

The strength properties and fracture toughness of the alloys wereestimated on conventional samples cut from different zones upon thethickness (t) of the semiproducts (¼ t and ½ t) in longitudinal (L orL-T) and short transverse (S or S-L) directions relative to fiberdirection.

Table 2 shows the results of testing for technological properties'estimation of the alloys of the present invention and the prior art.

The results given in this Table evidently show that the present alloy(compositions 2-9) 1.2-1.4 times exceeds the known alloy in flowabilityand technological plasticity.

Table 3 shows the properties of a central zone of the forgings with 150mm thickness made of the present alloy and the known alloy. One canevidently see from Table 3, that the present alloy 1.4-1.7 times exceedsthe known alloy in fracture toughness in L-T direction, and 1.2-1.4times—in S-T direction while the strength properties of both alloys arenearly the same. The best values of fracture toughness were defined onthe alloys 3-5, 7, 9 which had ratios Ti+2Zr≦0.3 and Si:Be≧2.

Table 4 shows the mechanical properties of semiproducts with differentthicknesses made of the present alloy and of the prior art alloy. Thedata of Table 4 shows that the present alloy as compared with the knownalloy, provides more uniform mechanical properties and fracturetoughness upon semiproduct' thickness what can especially be seen onlarge section samples with thickness of≧150 mm; said samples show 1.5-2times less reduction of strength properties and fracture toughness ascompared with the known alloy.

The present alloy having improved flowability, technological plasticity,fracture toughness, and also more uniform strength properties andfracture toughness upon thickness, allows to produce wide range ofsemiproducts (forged, extruded and rolled) practically of any shape anddimensions, especially of large section.

The large-sized integral articles having uniform properties made of thepresent alloy will allow to increase by 10-20% the weight efficiency ofthe structure due to reduction of riveted joint' number and will ensure15-20% increase of service reliability owing to improved fracturetoughness.

The improvement of technological properties of the present alloy willensure reduction of faulty production from said alloy, and use oflarge-sized semiproducts in aircraft structure will reduce labourintensity of assembling and will make the aircraft more economical by30-40%.

Producing and use of the present alloy and articles thereof do notdeteriorate environment from the ecological point of view.

TABLE 1 Compositions of experimental alloys No Compositions, mass. % n/nAlloy Zn Mg Cu Fe Si Zr Mn Cr Ti Be K Na Ca Al 1 Prior 6.7 2.0 1.4 0.10.05 0.11 0.02 0.02 — — — — — balance Art 2 Invention 8.0 2.5 1.3 0.250.2 0.2 0.1 0.05 0.1 0.05 0.01 0.01 0.01 balance 3 7.0 2.0 1.1 0.13 0.10.13 0.05 0.03 0.06 0.025 0.005 0.005 0.005 balance 4 6.35 0.5 0.8 0.060.01 0.07 0.001 0.001 0.03 0.0001 0.0001 0.0001 0.0001 balance 5 6.751.9 1.2 0.12 0.06 0.13 0.02 0.02 0.07 0.03 — — 0.008 balance 6 6.8 2.01.0 0.14 0.03 0.12 0.04 0.03 0.07 0.03 — 0.01 — balance 7 6.9 1.9 1.10.07 0.06 0.1 0.005 0.04 0.04 0.003 0.003 — — balance 8 7.0 2.0 1.1 0.130.03 0.13 0.05 0.02 0.05 0.042 0.005 — 0.01 balance 9 7.1 1.9 1.2 0.120.06 0.13 0.05 0.04 0.06 0.007 — 0.0005 0.0007 balance

TABLE 2 Technological properties of experimental alloys Technologicalplasticity, % upon upsetting upon Alloy Flowability, mm on a presstensile 1 270 70  85 Prior Art 2 360 89 135 3 370 94 140 4 370 97 138 5380 95 135 6 365 87 133 7 375 95 145 8 360 88 135 9 385 95 143

TABLE 3 Properties of hand forgings with 150 mm thickness in centralzone (1/2t) UTS, MPa YTS, MPa K_(1c)MPa {square root over (m)} Alloy LST L ST L-T S-L 1 490 467 420 405 31,5 26,2 Prior Art 2 528 515 485 47745,3 31,5 3 520 510 483 470 47,5 33,0 4 495 490 448 442 50,1 34,5 5 505490 450 440 47,7 34,3 6 508 491 451 443 45,6 32,9 7 509 489 455 450 47,034,0 8 512 493 450 448 46,9 32,0 9 502 495 455 450 47,5 34,5

TABLE 4 Mechanical properties of semiproducts with different thicknessesmade of experimental alloys Semi- Thickness YTS(L), MPa K_(1C)(L-T), MPa{square root over ( )} m YTS(ST), MPa K_(1C)(S-L), MPa {square root over( )} m Alloy product (t), mm ¼ t ½ t ¼ t ½ t ¼ t ½ t ½ t Prior Art Hand 60 470 468 — 37.1 — 445 30.1 Forging 100 465 455 37.2 34.2 440 438 29.3150 440 430 35.0 31.5 425 400 26.2 200 435 416 32.1 28.3 410 390 23.0Extrusion  60 470 468 — 36.3 — 461 32.1 130 455 430 35.7 33.1 440 41530.8 Suggested Hand  60 471 468 — 51.0 — 465 35.0 composition Forging100 465 462 49.6 49.1 460 455 34.8 (No 5) 150 455 450 48.3 47.7 445 44534.3 200 450 445 46.5 46.0 445 435 34.0 Extrusion  60 487 485 — 50.0 —479 36.7 130 485 485 45 48.0 483 480 36.0

We claim:
 1. High strength alloy of aluminum-zinc-magnesium-coppersystem comprising the following components (in mass. %): Zn 6.35-8.0 Si0.01-0.2 Mg 0.5-2.5 Fe 0.06-0.25 Cu 0.8-1.3 Zr 0.07-0.2 Cr 0.001-0.05 Ti0.03-0.1 Mn 0.001-0.1 Be 0.0001-0.05

and at least one element selected from the group consisting of: K0.0001-0.01, Na 0.0001-0.01 and Ca 0.0001-0.01; and Al-balance.


2. High strength aluminum-based alloy of claim 1, wherein the sumZr+2Ti≦0.3%.
 3. High strength aluminum-based alloy of claim 1, whereinthe ratio Si:Be≧2.
 4. An article made of a high strength aluminum-basedalloy said alloy comprising the following components (mass. %) Zn6.35-8.0 Si 0.01-0.2 Mg 0.5-2.5 Fe 0.06-0.25 Cu 0.8-1.3 Zr 0.07-0.2 Cr0.001-0.05 Ti 0.03-0.1 Mn 0.001-0.1 Be 0.0001-0.05

and at least one element selected from the group consisting of: K0.0001-0.01, Na 0.0001-0.01 and Ca 0.0001-0.01; and Al-balance.


5. The alloy of claim 1, wherein the alloy consists essentially of saidcomponents.
 6. The alloy of claim 1, wherein the alloy consists of saidcomponents.
 7. The alloy of claim 3, wherein the alloy consistsessentially of said components.
 8. The alloy of claim 3, wherein thealloy consists of said components.
 9. The article of claim 4, whereinthe alloy consists essentially of said components.
 10. The article ofclaim 4, wherein the alloy consists of said components.
 11. The alloy ofclaim 1, wherein the at least one element is K or Na.
 12. The alloy ofclaim 1, wherein the at least one element is Ca.
 13. The article ofclaim 4, wherein the at least one element is K or Na.
 14. The article ofclaim 4, wherein the at least one element is Ca.